Merge pull request #14529 from muxi/polish-connectivity-monitor

Refactor connectivity monitor on iOS

4 files changed, 165 insertions, 253 deletions

diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index ac4596da25..02492607cd 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -108,6 +108,9 @@ static NSString * const kBearerPrefix = @"Bearer ";
   // The dispatch queue to be used for enqueuing responses to user. Defaulted to the main dispatch
   // queue
   dispatch_queue_t _responseQueue;
+
+  // Whether the call is finished. If it is, should not call finishWithError again.
+  BOOL _finished;
 }
 
 @synthesize state = _state;
@@ -206,6 +209,8 @@ static NSString * const kBearerPrefix = @"Bearer ";
   } else {
     [_responseWriteable enqueueSuccessfulCompletion];
   }
+
+  [GRPCConnectivityMonitor unregisterObserver:self];
 }
 
 - (void)cancelCall {
@@ -214,9 +219,10 @@ static NSString * const kBearerPrefix = @"Bearer ";
 }
 
 - (void)cancel {
-  [self finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
-                                            code:GRPCErrorCodeCancelled
-                                        userInfo:@{NSLocalizedDescriptionKey: @"Canceled by app"}]];
+  [self maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                 code:GRPCErrorCodeCancelled
+                                             userInfo:@{NSLocalizedDescriptionKey: @"Canceled by app"}]];
+
   if (!self.isWaitingForToken) {
     [self cancelCall];
   } else {
@@ -224,6 +230,19 @@ static NSString * const kBearerPrefix = @"Bearer ";
   }
 }
 
+- (void)maybeFinishWithError:(NSError *)errorOrNil {
+  BOOL toFinish = NO;
+  @synchronized(self) {
+    if (_finished == NO) {
+      _finished = YES;
+      toFinish = YES;
+    }
+  }
+  if (toFinish == YES) {
+    [self finishWithError:errorOrNil];
+  }
+}
+
 - (void)dealloc {
   __block GRPCWrappedCall *wrappedCall = _wrappedCall;
   dispatch_async(_callQueue, ^{
@@ -250,11 +269,13 @@ static NSString * const kBearerPrefix = @"Bearer ";
   if (self.state == GRXWriterStatePaused) {
     return;
   }
-  __weak GRPCCall *weakSelf = self;
-  __weak GRXConcurrentWriteable *weakWriteable = _responseWriteable;
 
   dispatch_async(_callQueue, ^{
-    [weakSelf startReadWithHandler:^(grpc_byte_buffer *message) {
+    __weak GRPCCall *weakSelf = self;
+    __weak GRXConcurrentWriteable *weakWriteable = self->_responseWriteable;
+    [self startReadWithHandler:^(grpc_byte_buffer *message) {
+      __strong GRPCCall *strongSelf = weakSelf;
+      __strong GRXConcurrentWriteable *strongWriteable = weakWriteable;
       if (message == NULL) {
         // No more messages from the server
         return;
@@ -266,14 +287,14 @@ static NSString * const kBearerPrefix = @"Bearer ";
         // don't want to throw, because the app shouldn't crash for a behavior
         // that's on the hands of any server to have. Instead we finish and ask
         // the server to cancel.
-        [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
-                                                      code:GRPCErrorCodeResourceExhausted
-                                                  userInfo:@{NSLocalizedDescriptionKey: @"Client does not have enough memory to hold the server response."}]];
-        [weakSelf cancelCall];
+        [strongSelf maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                             code:GRPCErrorCodeResourceExhausted
+                                                         userInfo:@{NSLocalizedDescriptionKey: @"Client does not have enough memory to hold the server response."}]];
+        [strongSelf cancelCall];
         return;
       }
-      [weakWriteable enqueueValue:data completionHandler:^{
-        [weakSelf startNextRead];
+      [strongWriteable enqueueValue:data completionHandler:^{
+        [strongSelf startNextRead];
       }];
     }];
   });
@@ -333,12 +354,17 @@ static NSString * const kBearerPrefix = @"Bearer ";
     _requestWriter.state = GRXWriterStatePaused;
   }
 
-  __weak GRPCCall *weakSelf = self;
   dispatch_async(_callQueue, ^{
-    [weakSelf writeMessage:value withErrorHandler:^{
-      [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
-                                                    code:GRPCErrorCodeInternal
-                                                userInfo:nil]];
+    __weak GRPCCall *weakSelf = self;
+    [self writeMessage:value withErrorHandler:^{
+      __strong GRPCCall *strongSelf = weakSelf;
+      if (strongSelf != nil) {
+        [strongSelf maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                             code:GRPCErrorCodeInternal
+                                                         userInfo:nil]];
+        // Wrapped call must be canceled when error is reported to upper layers
+        [strongSelf cancelCall];
+      }
     }];
   });
 }
@@ -360,12 +386,15 @@ static NSString * const kBearerPrefix = @"Bearer ";
   if (errorOrNil) {
     [self cancel];
   } else {
-    __weak GRPCCall *weakSelf = self;
     dispatch_async(_callQueue, ^{
-      [weakSelf finishRequestWithErrorHandler:^{
-        [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
-                                                      code:GRPCErrorCodeInternal
-                                                  userInfo:nil]];
+      __weak GRPCCall *weakSelf = self;
+      [self finishRequestWithErrorHandler:^{
+        __strong GRPCCall *strongSelf = weakSelf;
+        [strongSelf maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                             code:GRPCErrorCodeInternal
+                                                         userInfo:nil]];
+        // Wrapped call must be canceled when error is reported to upper layers
+        [strongSelf cancelCall];
       }];
     });
   }
@@ -387,30 +416,37 @@ static NSString * const kBearerPrefix = @"Bearer ";
 }
 
 - (void)invokeCall {
+  __weak GRPCCall *weakSelf = self;
   [self invokeCallWithHeadersHandler:^(NSDictionary *headers) {
     // Response headers received.
-    self.responseHeaders = headers;
-    [self startNextRead];
+    __strong GRPCCall *strongSelf = weakSelf;
+    if (strongSelf) {
+      strongSelf.responseHeaders = headers;
+      [strongSelf startNextRead];
+    }
   } completionHandler:^(NSError *error, NSDictionary *trailers) {
-    self.responseTrailers = trailers;
+    __strong GRPCCall *strongSelf = weakSelf;
+    if (strongSelf) {
+      strongSelf.responseTrailers = trailers;
 
-    if (error) {
-      NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
-      if (error.userInfo) {
-        [userInfo addEntriesFromDictionary:error.userInfo];
-      }
-      userInfo[kGRPCTrailersKey] = self.responseTrailers;
-      // TODO(jcanizales): The C gRPC library doesn't guarantee that the headers block will be
-      // called before this one, so an error might end up with trailers but no headers. We
-      // shouldn't call finishWithError until ater both blocks are called. It is also when this is
-      // done that we can provide a merged view of response headers and trailers in a thread-safe
-      // way.
-      if (self.responseHeaders) {
-        userInfo[kGRPCHeadersKey] = self.responseHeaders;
+      if (error) {
+        NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
+        if (error.userInfo) {
+          [userInfo addEntriesFromDictionary:error.userInfo];
+        }
+        userInfo[kGRPCTrailersKey] = strongSelf.responseTrailers;
+        // TODO(jcanizales): The C gRPC library doesn't guarantee that the headers block will be
+        // called before this one, so an error might end up with trailers but no headers. We
+        // shouldn't call finishWithError until ater both blocks are called. It is also when this is
+        // done that we can provide a merged view of response headers and trailers in a thread-safe
+        // way.
+        if (strongSelf.responseHeaders) {
+          userInfo[kGRPCHeadersKey] = strongSelf.responseHeaders;
+        }
+        error = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
       }
-      error = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
+      [strongSelf maybeFinishWithError:error];
     }
-    [self finishWithError:error];
   }];
   // Now that the RPC has been initiated, request writes can start.
   @synchronized(_requestWriter) {
@@ -439,16 +475,8 @@ static NSString * const kBearerPrefix = @"Bearer ";
     // TODO(jcanizales): Check this on init.
     [NSException raise:NSInvalidArgumentException format:@"host of %@ is nil", _host];
   }
-  _connectivityMonitor = [GRPCConnectivityMonitor monitorWithHost:host];
-  __weak typeof(self) weakSelf = self;
-  void (^handler)(void) = ^{
-    typeof(self) strongSelf = weakSelf;
-    [strongSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
-                                                    code:GRPCErrorCodeUnavailable
-                                                userInfo:@{ NSLocalizedDescriptionKey : @"Connectivity lost." }]];
-  };
-  [_connectivityMonitor handleLossWithHandler:handler
-                      wifiStatusChangeHandler:nil];
+  [GRPCConnectivityMonitor registerObserver:self
+                                   selector:@selector(connectivityChanged:)];
 }
 
 - (void)startWithWriteable:(id<GRXWriteable>)writeable {
@@ -512,4 +540,12 @@ static NSString * const kBearerPrefix = @"Bearer ";
   }
 }
 
+- (void)connectivityChanged:(NSNotification *)note {
+  [self maybeFinishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                 code:GRPCErrorCodeUnavailable
+                                             userInfo:@{ NSLocalizedDescriptionKey : @"Connectivity lost." }]];
+  // Cancel underlying call upon this notification
+  [self cancelCall];
+}
+
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
index cb55e46d70..394d21792d 100644
--- a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
+++ b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
@@ -19,44 +19,30 @@
 #import <Foundation/Foundation.h>
 #import <SystemConfiguration/SystemConfiguration.h>
 
-@interface GRPCReachabilityFlags : NSObject
-
-+ (nonnull instancetype)flagsWithFlags:(SCNetworkReachabilityFlags)flags;
-
-/**
- * One accessor method to query each of the different flags. Example:
-
-@property(nonatomic, readonly) BOOL isCell;
-
- */
-#define GRPC_XMACRO_ITEM(methodName, FlagName) \
-@property(nonatomic, readonly) BOOL methodName;
-
-#include "GRPCReachabilityFlagNames.xmacro.h"
-#undef GRPC_XMACRO_ITEM
-
-@property(nonatomic, readonly) BOOL isHostReachable;
-@end
-
+typedef NS_ENUM(NSInteger, GRPCConnectivityStatus) {
+  GRPCConnectivityUnknown = 0,
+  GRPCConnectivityNoNetwork = 1,
+  GRPCConnectivityCellular = 2,
+  GRPCConnectivityWiFi = 3,
+};
+
+extern NSString * _Nonnull kGRPCConnectivityNotification;
+
+// This interface monitors OS reachability interface for any network status
+// change. Parties interested in these events should register themselves as
+// observer.
 @interface GRPCConnectivityMonitor : NSObject
 
-+ (nullable instancetype)monitorWithHost:(nonnull NSString *)hostName;
-
 - (nonnull instancetype)init NS_UNAVAILABLE;
 
-/**
- * Queue on which callbacks will be dispatched. Default is the main queue. Set it before calling
- * handleLossWithHandler:.
- */
-// TODO(jcanizales): Default to a serial background queue instead.
-@property(nonatomic, strong, null_resettable) dispatch_queue_t queue;
-
-/**
- * Calls handler every time the connectivity to this instance's host is lost. If this instance is
- * released before that happens, the handler won't be called.
- * Only one handler is active at a time, so if this method is called again before the previous
- * handler has been called, it might never be called at all (or yes, if it has already been queued).
- */
-- (void)handleLossWithHandler:(nullable void (^)(void))lossHandler
-      wifiStatusChangeHandler:(nullable void (^)(void))wifiStatusChangeHandler;
+// Register an object as observer of network status change. \a observer
+// must have a notification method with one parameter of type
+// (NSNotification *) and should pass it to parameter \a selector. The
+// parameter of this notification method is not used for now.
++ (void)registerObserver:(_Nonnull id)observer
+                selector:(_Nonnull SEL)selector;
+
+// Ungegister an object from observers of network status change.
++ (void)unregisterObserver:(_Nonnull id)observer;
+
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
index c8e10dd75f..7f31c7e23e 100644
--- a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
+++ b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
@@ -18,175 +18,74 @@
 
 #import "GRPCConnectivityMonitor.h"
 
-#pragma mark Flags
+#include <netinet/in.h>
 
-@implementation GRPCReachabilityFlags {
-  SCNetworkReachabilityFlags _flags;
-}
+NSString *kGRPCConnectivityNotification = @"kGRPCConnectivityNotification";
 
-+ (instancetype)flagsWithFlags:(SCNetworkReachabilityFlags)flags {
-  return [[self alloc] initWithFlags:flags];
-}
+static SCNetworkReachabilityRef reachability;
+static GRPCConnectivityStatus currentStatus;
 
-- (instancetype)initWithFlags:(SCNetworkReachabilityFlags)flags {
-  if ((self = [super init])) {
-    _flags = flags;
+// Aggregate information in flags into network status.
+GRPCConnectivityStatus CalculateConnectivityStatus(SCNetworkReachabilityFlags flags) {
+  GRPCConnectivityStatus result = GRPCConnectivityUnknown;
+  if (((flags & kSCNetworkReachabilityFlagsReachable) == 0) ||
+      ((flags & kSCNetworkReachabilityFlagsConnectionRequired) != 0)) {
+    return GRPCConnectivityNoNetwork;
   }
-  return self;
-}
-
-/*
- * One accessor method implementation per flag. Example:
-
-- (BOOL)isCell { \
-  return !!(_flags & kSCNetworkReachabilityFlagsIsWWAN); \
-}
-
- */
-#define GRPC_XMACRO_ITEM(methodName, FlagName) \
-- (BOOL)methodName { \
-  return !!(_flags & kSCNetworkReachabilityFlags ## FlagName); \
-}
-#include "GRPCReachabilityFlagNames.xmacro.h"
-#undef GRPC_XMACRO_ITEM
-
-- (BOOL)isHostReachable {
-  // Note: connectionOnDemand means it'll be reachable only if using the CFSocketStream API or APIs
-  // on top of it.
-  // connectionRequired means we can't tell until a connection is attempted (e.g. for VPN on
-  // demand).
-  return self.reachable && !self.interventionRequired && !self.connectionOnDemand;
-}
-
-- (NSString *)description {
-  NSMutableArray *activeOptions = [NSMutableArray arrayWithCapacity:9];
-
-  /*
-   * For each flag, add its name to the array if it's ON. Example:
-
-  if (self.isCell) {
-    [activeOptions addObject:@"isCell"];
+  result = GRPCConnectivityWiFi;
+#if TARGET_OS_IPHONE
+  if (flags & kSCNetworkReachabilityFlagsIsWWAN) {
+    return result = GRPCConnectivityCellular;
   }
-
-   */
-  #define GRPC_XMACRO_ITEM(methodName, FlagName) \
-    if (self.methodName) {                       \
-      [activeOptions addObject:@ #methodName];   \
-    }
-  #include "GRPCReachabilityFlagNames.xmacro.h"
-  #undef GRPC_XMACRO_ITEM
-
-  return activeOptions.count == 0 ? @"(none)" : [activeOptions componentsJoinedByString:@", "];
-}
-
-- (BOOL)isEqual:(id)object {
-  return [object isKindOfClass:[GRPCReachabilityFlags class]] &&
-      _flags == ((GRPCReachabilityFlags *)object)->_flags;
-}
-
-- (NSUInteger)hash {
-  return _flags;
-}
-@end
-
-#pragma mark Connectivity Monitor
-
-// Assumes the third argument is a block that accepts a GRPCReachabilityFlags object, and passes the
-// received ones to it.
-static void PassFlagsToContextInfoBlock(SCNetworkReachabilityRef target,
-                                        SCNetworkReachabilityFlags flags,
-                                        void *info) {
-  #pragma unused (target)
-  // This can be called many times with the same info. The info is retained by SCNetworkReachability
-  // while this function is being executed.
-  void (^handler)(GRPCReachabilityFlags *) = (__bridge void (^)(GRPCReachabilityFlags *))info;
-  handler([[GRPCReachabilityFlags alloc] initWithFlags:flags]);
+#endif
+  return result;
 }
 
-@implementation GRPCConnectivityMonitor {
-  SCNetworkReachabilityRef _reachabilityRef;
-  GRPCReachabilityFlags *_previousReachabilityFlags;
-}
+static void ReachabilityCallback(
+    SCNetworkReachabilityRef target, SCNetworkReachabilityFlags flags, void* info) {
+  GRPCConnectivityStatus newStatus = CalculateConnectivityStatus(flags);
 
-- (nullable instancetype)initWithReachability:(nullable SCNetworkReachabilityRef)reachability {
-  if (!reachability) {
-    return nil;
+  if (newStatus != currentStatus) {
+    [[NSNotificationCenter defaultCenter] postNotificationName:kGRPCConnectivityNotification
+                                                        object:nil];
+    currentStatus = newStatus;
   }
-  if ((self = [super init])) {
-    _reachabilityRef = CFRetain(reachability);
-    _queue = dispatch_get_main_queue();
-    _previousReachabilityFlags = nil;
-  }
-  return self;
 }
 
-+ (nullable instancetype)monitorWithHost:(nonnull NSString *)host {
-  const char *hostName = host.UTF8String;
-  if (!hostName) {
-    [NSException raise:NSInvalidArgumentException
-                format:@"host.UTF8String returns NULL for %@", host];
-  }
-  SCNetworkReachabilityRef reachability =
-      SCNetworkReachabilityCreateWithName(NULL, hostName);
+@implementation GRPCConnectivityMonitor
 
-  GRPCConnectivityMonitor *returnValue = [[self alloc] initWithReachability:reachability];
-  if (reachability) {
-    CFRelease(reachability);
-  }
-  return returnValue;
-}
++ (void)initialize {
+  if (self == [GRPCConnectivityMonitor self]) {
+    struct sockaddr_in addr = {0};
+    addr.sin_len = sizeof(addr);
+    addr.sin_family = AF_INET;
+    reachability = SCNetworkReachabilityCreateWithAddress(NULL, (struct sockaddr *)&addr);
+    currentStatus = GRPCConnectivityUnknown;
 
-- (void)handleLossWithHandler:(nullable void (^)(void))lossHandler
-      wifiStatusChangeHandler:(nullable void (^)(void))wifiStatusChangeHandler {
-  __weak typeof(self) weakSelf = self;
-  [self startListeningWithHandler:^(GRPCReachabilityFlags *flags) {
-    typeof(self) strongSelf = weakSelf;
-    if (strongSelf) {
-      if (lossHandler && !flags.reachable) {
-        lossHandler();
-#if TARGET_OS_IPHONE
-      } else if (wifiStatusChangeHandler &&
-                 strongSelf->_previousReachabilityFlags &&
-                 (flags.isWWAN ^
-                  strongSelf->_previousReachabilityFlags.isWWAN)) {
-        wifiStatusChangeHandler();
-#endif
-      }
-      strongSelf->_previousReachabilityFlags = flags;
+    SCNetworkConnectionFlags flags;
+    if (SCNetworkReachabilityGetFlags(reachability, &flags)) {
+      currentStatus = CalculateConnectivityStatus(flags);
     }
-  }];
-}
 
-- (void)startListeningWithHandler:(void (^)(GRPCReachabilityFlags *))handler {
-  // Copy to ensure the handler block is in the heap (and so can't be deallocated when this method
-  // returns).
-  void (^copiedHandler)(GRPCReachabilityFlags *) = [handler copy];
-  SCNetworkReachabilityContext context = {
-    .version = 0,
-    .info = (__bridge void *)copiedHandler,
-    .retain = CFRetain,
-    .release = CFRelease,
-  };
-  // The following will retain context.info, and release it when the callback is set to NULL.
-  SCNetworkReachabilitySetCallback(_reachabilityRef, PassFlagsToContextInfoBlock, &context);
-  SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, _queue);
-}
-
-- (void)stopListening {
-  // This releases the block on context.info.
-  SCNetworkReachabilitySetCallback(_reachabilityRef, NULL, NULL);
-  SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, NULL);
+    SCNetworkReachabilityContext context = {0, (__bridge void *)(self), NULL, NULL, NULL};
+    if (!SCNetworkReachabilitySetCallback(reachability, ReachabilityCallback, &context) ||
+        !SCNetworkReachabilityScheduleWithRunLoop(
+            reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes)) {
+      NSLog(@"gRPC connectivity monitor fail to set");
+    }
+  }
 }
 
-- (void)setQueue:(dispatch_queue_t)queue {
-  _queue = queue ?: dispatch_get_main_queue();
++ (void)registerObserver:(_Nonnull id)observer
+                selector:(SEL)selector {
+  [[NSNotificationCenter defaultCenter] addObserver:observer
+                                           selector:selector
+                                               name:kGRPCConnectivityNotification
+                                             object:nil];
 }
 
-- (void)dealloc {
-  if (_reachabilityRef) {
-    [self stopListening];
-    CFRelease(_reachabilityRef);
-  }
++ (void)unregisterObserver:(_Nonnull id)observer {
+  [[NSNotificationCenter defaultCenter] removeObserver:observer];
 }
 
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCHost.m b/src/objective-c/GRPCClient/private/GRPCHost.m
index 71b57cf1f6..8568e334dd 100644
--- a/src/objective-c/GRPCClient/private/GRPCHost.m
+++ b/src/objective-c/GRPCClient/private/GRPCHost.m
@@ -37,12 +37,6 @@ NS_ASSUME_NONNULL_BEGIN
 
 static NSMutableDictionary *kHostCache;
 
-// This connectivity monitor flushes the host cache when connectivity status
-// changes or when connection switch between Wifi and Cellular data, so that a
-// new call will use a new channel. Otherwise, a new call will still use the
-// cached channel which is no longer available and will cause gRPC to hang.
-static GRPCConnectivityMonitor *connectivityMonitor = nil;
-
 @implementation GRPCHost {
   // TODO(mlumish): Investigate whether caching channels with strong links is a good idea.
   GRPCChannel *_channel;
@@ -90,17 +84,7 @@ static GRPCConnectivityMonitor *connectivityMonitor = nil;
       kHostCache[address] = self;
       _compressAlgorithm = GRPC_COMPRESS_NONE;
     }
-    // Keep a single monitor to flush the cache if the connectivity status changes
-    // Thread safety guarded by @synchronized(kHostCache)
-    if (!connectivityMonitor) {
-      connectivityMonitor =
-      [GRPCConnectivityMonitor monitorWithHost:hostURL.host];
-      void (^handler)(void) = ^{
-        [GRPCHost flushChannelCache];
-      };
-      [connectivityMonitor handleLossWithHandler:handler
-                         wifiStatusChangeHandler:handler];
-    }
+    [GRPCConnectivityMonitor registerObserver:self selector:@selector(connectivityChange:)];
   }
   return self;
 }
@@ -281,6 +265,13 @@ static GRPCConnectivityMonitor *connectivityMonitor = nil;
   }
 }
 
+// Flushes the host cache when connectivity status changes or when connection switch between Wifi
+// and Cellular data, so that a new call will use a new channel. Otherwise, a new call will still
+// use the cached channel which is no longer available and will cause gRPC to hang.
+- (void)connectivityChange:(NSNotification *)note {
+  [GRPCHost flushChannelCache];
+}
+
 @end
 
 NS_ASSUME_NONNULL_END